A wavelength path reallocation method in a path reallocation apparatus for reallocating a wavelength path set in a communication network, including: a wavelength path designing step in which a wavelength path designing unit designs a reallocation destination wavelength path by performing calculation such that the number of use frequency regions in the communication network becomes smaller than a corresponding value before reallocation; and a wavelength path setting step in which a wavelength path setting unit changes a reallocation target wavelength path to the reallocation destination wavelength path by using free wavelength.